This invention relates to nuclear reactors and is primarily directed to liquid metal cooled nuclear reactors.
One known kind of liquid metal cooled nuclear reactor comprises a nuclear reactor core submerged in a pool of liquid sodium contained by a primary vessel within a concrete vault. The core is supported on a diagrid and enclosed by a shroud or core tank and coolant is circulated from outside the core tank by a pump upwardly through the reactor core thence to a heat exchanger from which the coolant is discharged back to the region of the pool which is outside the core tank. The temperature of the pool outside the core tank is approximately 400.degree. C. whilst that inside the core tank is approximately 600.degree. C.
To reduce heat transfer from the inner to the outer region of the pool and to protect the wall of the core tank from the stress effects of such a large temperature differential it is necessary to provide thermal insulation for the inner wall, that is, the hot wall, of the tank. In the known kind of liquid metal cooled nuclear reactor the insulation has been of the passive kind, that is, cladding of the kind which has a low heat transfer characteristic and has taken such form as a gas filled quilted envelope made from thin stainless steel material. However, such insulation is unreliable and requires testing for gas tightness by complex techniques. In a recently proposed construction of liquid metal nuclear reactor passive insulation for the wall of the core tank comprises a cladding layer of closely packed and inter-sealed stainless steel blocks and a stainless steel membrane spaced from the layer of blocks. The membrane is provided with a network of intersecting corrugations to accommodate thermal expansion of the membrane but it is feared that the complex stresses induced at the knots of the orthogonal corrugations will give rise to unreliability.
It is an object of the present invention to develop insulation means for the core tank of a liquid metal cooled nuclear reactor wherein the liquid metal coolant plays a more active part in the insulation and thereby reduces the complexity of manufacture and improves the reliability of the insulation.